Mid-Altitude Radial Convergence (MARC)

MARC is a Doppler velocity signature that serves as a precursor to the initial onset of damaging downburst winds in a squall line. Preliminary squall line case studies have shown a good correlation between values of MARC velocity differentials of at least 25 to 30 m/s (50 kts or greater) to successive damaging surface wind gusts. Strong mid-altitude radial convergence or MARC is often observed along the forward flank or leading edge of an organized bowing convective system. The region of strong outbound velocities signifies a component of the system's updraft current and front-to-rear flow (when viewing a storm approaching from the west). The region of strong inbound velocities depicts the storm's convective-scale downdrafts and origins of the mesoscale rear-inflow jet. These enhanced velocity differentials or areas of strong radial convergence are usually located in, or just ahead of the high reflectivity cores along the leading edge of the convective line. (See the example to the right.)

Two of the major advantages of using the MARC signature as a precursor to damaging winds include: 1) potentially longer lead times (typically from 10 to 30 min before the first report of severe wind gusts), and 2) the relatively large distance from the radar at which one can detect this signature. One major limitation of using this signature occurs when the mid-level convergence is orthogonal to the radar beam, and velocities and convergence values are greatly underestimated.

Summary of MARC Characteristics:

Horizontal Extent
One to three locally enhanced convergent areas (velocity differentials) are found embedded within a larger region of convergence extending from 60 to 120 km in length
Depth
Average of 6.2 km (from 3 to 9 km in height), with the maximum convergence found in the mid-levels of the storm (between 5 and 5.5 km in height)
Width
2 to 6 km
Magnitude
Typical velocity differences of 25 to 50 m/s (50 to 100 kts) with actual convergence values of 2.5 x10-2 to
5.6 x 10-3

For more information on the MARC signature refer to Schmocker et al. (1996).